In industrial settings, control systems are used to monitor and control inventories of industrial and chemical processes, and the like. Typically, the control system performs these functions using field devices distributed at key locations in the industrial process and coupled to the control circuitry in the control room by control loop. The term “field device” refers to any device that performs a function in a distributed control or process monitoring system, including all devices used in the measurement, control and monitoring of industrial processes.
Field devices are used by the process control and measurement industry for a variety of purposes. Usually, such field devices have a field-hardened enclosure so that they can be installed outdoors in relatively rugged environments and able to withstand climatological extremes of temperature, humidity, vibration, mechanical shock, et cetera. These field devices also can typically operate on relatively low power. For example, field devices are currently available that receive all of their operating power from a known 4-20 mA loop.
Some field devices include a transducer. A transducer is understood to mean either a device that generates an output signal based on a physical input or that generates a physical output based on an input signal. Typically, a transducer transforms an input signal into an output having a different form. Types of transducers include various analytical equipment, pressure sensors, thermistors, thermocouples, strain gauges, flow transmitters, positioners, actuators, solenoids, indicator lights, and others.
Analog field devices have been connected to the control room by two-wire process control current loops, with each device typically connected to the control room by a single two-wire control loop. Typically, a voltage differential is maintained between the two wires within a range from about 12-45 volts. Some analog field devices transmit a signal to the control room by modulating the current running through the current loop to a current proportional to the sensed process variable. Other analog field devices can perform an action based upon a magnitude of a current signal set by a control room and flowing through the loop.
In addition to, or in the alternative, the process control loop can carry digital signals used for communication with field devices. Digital communication provides much more detail about the connected device than analog communication. Moreover, digital field devices also do not require separate wiring for each such field device. Field devices that communicate digitally can respond to and communicate selectively with the control room and/or other field devices. Further, such devices can provide additional signaling such as diagnostics and/or alarms.
Known process communication methods include simply using a 4-20 mA analog communication loop, hybrid protocols, such as the Highway Addressable Remote Transducer (HART®) standard, or all-digital protocols such as the FOUNDATION™ fieldbus standard.
Over time, physical elements within, or connected to, a process device may change and/or age. These changes can be due to exposure of the field device to external forces such as temperature or pressure extremes, corrosion, et cetera. Thus, it is necessary, from time to time, to calibrate field devices. This is often accomplished using a calibrator. The calibrator steps the maintenance technician through a series of steps which involve applying a known physical input (e.g. pressure, temperature, et cetera) and then recording the value that the field device outputs in response to the known physical input. Some known calibrators can accept the download of a “route” from an asset management system. This route lists the field devices that are to be calibrated and the steps that are to be performed for each device. At the end of the route, all of calibration data acquired by the calibrator for the field devices can be uploaded back into an asset management system. Using this process ensures complete, accurate documentation of all calibration activity associated with each field device. Calibrators that acquire data relative to calibrations, and temporarily store such data for later upload to an asset management system are known as documenting calibrators.